The Pupils:An Intro

The entrance and exit pupils of a lens are a tough subject matter to comprehend. The subject is even harder to find good simple written discussions on. To start: What are the pupils? To put it as simply as possible, the pupils are the apparent size and position of the aperture of the lens when viewed from the front or the back.

When you look at the aperture of the lens from the front or the back of the lens you are seeing the aperture by way of the intervening lenses. Those lenses will tend to change the size and position of the aperture in relation to its real position in space. The entrance pupil is the aperture when viewed through the front of the lens and the exit pupil is the aperture when viewed through the back of the lens. The entrance and exit pupils can be wildly different from each other in size and position depending upon the lens construction.

So, why should you care about the pupils? For me, I don't really need to know the exact position or size of the pupils, but I do find it useful to understand that the pupils are where the work of the aperture is accomplished.